Why can my instance initializer block reference a field before it is declared?

Question

My understanding is that you cannot reference a variable before it has been declared, and that all code (including instance initializers) that is within the body of a class,

Answer 1

The order of declaration is not important. You could also write:

public  class WhyIsThisOk {

    {
        a = 5;
    }

    public WhyIsThisOk() {
    }

    public static void main(String[] args) {
        System.out.println(new WhyIsThisOk().a);
    }

    int a = 10;
}

Important is, that the compiler copies (top down) first a=5 and then a=10 into constructor so it looks like:

public WhyIsThisOk() {
    a = 5;
    a = 10;
}

Finally look at this example:

public class WhyIsThisOk {

    {
        a = get5();
    }

    public WhyIsThisOk() {
        a = get7();
    }

    public static void main(String[] args) {
        System.out.println(new WhyIsThisOk().a);
    }

    int a = get10();

    public static int get5() {
        System.out.println("get5 from: " + new Exception().getStackTrace()[1].getMethodName());
        return 5;
    }

    public static int get7() {
        System.out.println("get7 from: " + new Exception().getStackTrace()[1].getMethodName());
        return 7;
    }

    public static int get10() {
        System.out.println("get10 from: " + new Exception().getStackTrace()[1].getMethodName());
        return 10;
    }
}

Output is:

get5 from: 
get10 from: 
get7 from: 
7